Young'un amongst oldies by any chance?
I grew up in a small town. I don't know if it was a thing there or just a peculiarity amongst my family and friends but you rarely knocked, you just kind of yelled a greeting (which is usually "knock knock" in my case XD) and if someone answered you from the bowels of the house then you waltzed on in XD
I like that. That sounds like the town in the 'far' east of Holland, where my grandparents used to live. Doors were never locked and there was no need to make appointments. You wanted to see someone? you just went over. If the peepz were - indeed - around, you were always welcome to stay and enjoy whatever food and drinks there were around. Now I think of it, the backdoor was always open :>)